1. Field of the Invention
The present invention relates to a visible-light communications technique that achieves bidirectional visible-light communication.
2. Description of the Related Art
In recent years, a visible-light communications technology has been developed which utilizes the visible light emitted from a light source such as a light emitting diode (hereinafter referred to as “LED,” on some occasions). To accomplish bidirectional communication using visible light, it is proposed that a hybrid system should be utilized in place of a system wherein the transmitting side and the receiving side have a light source each. In the hybrid system, visible light communication is combined with infrared-ray communication or radio-wave communication.
However, such hybrid system may fail to achieve fully the advantage of visible light communication, i.e., that which takes place within the visible light spectrum. In such system, both the transmitting side and the receiving side have a light source each and therefore need a power supply each, for the light source. Inevitably, it is difficult to apply this system to communications apparatuses that have but a limited power supply, such as mobile terminals.
In view of this, a single-light-source system has been proposed, which has two communication apparatuses and in which one apparatus has a light source that emits visible light and the other apparatus reflects the visible light, thereby transmitting data to the first-mentioned apparatus. (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-221747.)
The single-light source system in which one of the apparatuses need not have a light-source power supply can accomplish bidirectional visible-light communication using mobile terminals that are designed to operate with low power consumption.
In the single-light-source system, wherein the visible light beam emitted from one communications apparatus is superimposed with transmission data, one of the communication apparatuses cannot transmit data merely by using a reflected light beam obtained from the visible light beam. This is because the visible light emitted from the transmitting side is superimposed with the transmission data. Hence, a visible-light communications system that uses a single light source must be subjected to some restrictions, such as the use of a light beam only while no transmission data is being superimposed on the visible light beam.